The present invention relates to the field of internal combustion engines, and in particular to methods of increasing the efficiency of the cylinder head charging, fuel burn and subsequent exhaust during a combustion cycle.
Classic internal combustion engines have two valves per cylinder. One valve, the intake valve, admits a fuel/air mixture into the cylinder head. The other, exhaust valve, permits expulsion of combustion products from the cylinder head and thence to an exhaust. During combustion events the cylinder head valves are generally both closed. Once combustion has occurred and the piston head has been forced downwards in the cylinder, the exhaust valve is opened and burnt mixture is expelled from the cylinder head by return motion of the piston head. The exhaust valve is closed as the piston reaches the top of the cylinder. The intake valve is opened so that movement of the piston back down in the cylinder sucks air/fuel mixture into the cylinder head, ready for the next spark ignition to create combustion of the air/fuel mixture.
The common internal combustion engine can be likened to a complex thermodynamic air pump. The greater the rate that an engine can pass air, the greater the potential for generating more power. However, air consumption alone does not guarantee power. Unless the air/fuel charge entering the cylinder head is burned effectively, the torque and consequently the power output of the engine as a whole will not reach its full potential.
One way of increasing the efficiency of air flow through an engine is increasing the number of valves. Four valve per cylinder head engines are common and permit a greater through flow for a given valve dimension. There are two intake valves and two exhaust valves, which provide increased air flow by providing a greater xe2x80x9ccurtain areaxe2x80x9d for a given valve diameter. Curtain area is the cylindrical area swept by the valve head between its closed an open configurations, in other words the multiple of valve lift and valve circumference. For a constant valve head area, a cylinder head formed with one intake valve head will have a reduced curtain area compared to a cylinder head formed with two smaller intake valve heads having the same total valve head diameter. This is because the total circumference of the two smaller heads will be greater than that of one large valve head. The advantage of the greater circumference of the two smaller valves persists through out the valve lift range from closed up to a lift equal to 0.25 of the valve diameter. From this point on, the advantage over a single valve diminishes until the single valve reaches a lift of 0.25 of the valve diameter.
The four-valve cylinder head has a further advantage. The valve area which can be accommodated in a circular section cylinder head is greater with four valves than with two valves. The previously discussed circumferential curtain advantage plus this area advantage gives the four-valve head its superior air breathing qualities and consequently it""s potential to produce greater power.
However, One of the principle functional disadvantages of a 4-valve cylinder head as compared with a 2-valve head is that the 4-valve head lacks much of the air/fuel mixture motion generated during induction of the fresh charge into the 4-valve head. FIG. 1 illustrates the typical mixture motion generated by a well-designed 2-valve cylinder head 1 provided in a roof thereof with an inlet valve 3 and an exhaust valve 4. As the charge (indicated by arrows) enters the combustion chamber 2, so a swirling motion is set up. This motion, if aggressive enough, persists through out the compression stroke and on into the combustion process. The swirl effectively speeds the combustion process thus producing a higher-pressure rise and consequently more torque. This mixture motion typically allows a 2 valve per cylinder engine to produce more low rpm torque for a given size of engine than its 4-valve counterpart. At low engine speed the fact that a 2-valve engine may have less breathing area presented to the cylinder is of little consequence because the time available to fill the cylinder is more than adequate. As rpm rises so the need for greater breathing capability increases. A 4 valve per cylinder automotive engine typically exhibits its advantage in terms of breathing (or volumetric efficiency defined as the amount of air drawn into the cylinder divided by the cylinder displacement) at above about 4000 to 4500 rpm.
FIG. 1 demonstrates that the swirl of a two-valve engine""s cylinder head is principally an inherent characteristic of the basic design. A 4-valve head is not without its own characteristic motion. This motion is most commonly known as tumble. Tumble is generated because the intake valve is offset to one side of the cylinder head. Hence air/fuel mixture entering the head is presented with a large free space at the other side of the cylinder head. The mixture admitted is predominantly drawn over the top of the open valve head towards the other side of the cylinder head. The mixture then xe2x80x98tumblesxe2x80x99 down towards the piston head, back across the piston head and back up towards the intake valves. Although tumble helps produce an accelerated combustion process at low rpm, it generally fails to match the effectiveness of the two-valve design. Four-valve cylinder heads usually have inclined valves, in which the plane occupied by the valve heads at any point in time is angled with respect to a top face of the cylinder head (or the engine block face). This configuration leads to another disadvantage of typical 4 valve cylinder heads. This is the phenomenon of xe2x80x98cross flowxe2x80x99. Cross flow arises where fuel/air mixture entering the cylinder head travels directly from the intake valve to the exhaust valve and out of the cylinder head. This can, of course, only occur if both intake and exhaust valves are open at the same time. This happens between a period starting just before top dead centre at the end of the exhaust stroke to just after top dead centre on the intake stroke. During this period, when both valves are open, there is a tendency for some of the fresh charge to exit the cylinder via the still open exhaust valve.
Cross flow causes an increase in fuel consumption and unwanted exhaust emissions, mostly in the form of unburned hydrocarbons. The effects of cross flow are normally countered by shortening the duration of the valve opening events to cut the overlap. However, if duration is shortened as far as is often needed, much of the power advantage of a 4-valve design may be lost.
One method of generating swirl in 4-valve engines is to disable one valve while the engine is at lower RPM. FIG. 2 is a schematic view from above of a 4-valve cylinder head 5, provided with two inlet valves 6,7 and two exhaust valves 8,9. The effect of blocking one inlet valve 7 is to cause the 4-valve engine to function as a 2-valve engine at low rpm. The second intake 7 port is typically deactivated by means of a closed butterfly valve 10. As engine rpm rises, the need for aggressive mixture motion decreases but the requirement for strong airflow increases. Hence at higher revolutions, usually in the region of 3500 rpm, the butterfly opens and allows the second valve port to supply the engine""s air demand.
The foregoing method is not the only technique for introducing swirl into a 4-valve engine. Other techniques are used in the design and manufacture of Diesel engines that need high swirl values for effective combustion. A significant problem with the known methods of inducing swirl in 4-valve engines is the requirement for a complex mechanical arrangement for opening and closing the butterfly valve according to the threshold rpm for efficient operation.
A recent development, which is a simplification of 4 valve per cylinder engines, is the 3 valve per cylinder engine. In such engines each cylinder has two intake valves and a single exhaust valve. This type of cylinder head valve arrangement does not, however, generate any significant swirl.
GB-A-2215777 (Honda) discloses an engine having differently sized intake valves, with a helical feed angle to develop swirl. The smaller intake valve is provided with a throttle which is activated at low revs. The technology relates to direct injection diesel engines. One intake valve is stated to be small than the other in order to minimize flow xe2x80x98shockxe2x80x99 on closure of the smaller valve at low revs.
A number of patent documents have disclosed differential sizing of intake valves for producing combustion chamber swirl. In addition the use of differential valve timing is known for the same purpose. For example:
U.S. Pat. No. 5,007,392 (Honda) discloses an engine in which there are differently-sized intake valves and differently-sized exhaust valves, primarily for improving volumetric engine and through-flow efficiency, and in addition for generating swirl. Variable valve timing is also used to impart swirl.
WO 90/05842 (Johannes) discloses an engine in which intake valves have respectively lateral and oblique entry directions, in order to generate a vortex. The respective cross-sections of the intake valves differ in order to obtain different velocities through the ports. The smaller, high velocity, port is opened first at low engine speeds. The port throats are provided with throttles to allow incremental increasing opening of the ports so as to produce the optimum cylinder charge at any revs. In the case of multiple intake valves, the intake valve most remote from the exhaust valve is opened first, so as to reduce cross-flow.
JP-A-59-099026 (Mazda) discloses a cylinder head in which the intake valves are differently sized so as to induce swirl. The exhaust valves are symmetrically differently sized, the valve timing overlap between large intake and large exhaust valves being greater than that between the smaller valves.
JP-A-57-091320 (Suzuki) discloses a cylinder head in which two differently sized intake ports are provided, the sizing imparting swirl. The invention relates to the use of a single rocker arm to open both intake valve heads by use of a rotating cam acting on the rocker arm.
The present invention seeks to provide, inter alia, further methods to generate swirl and reduce cross flow in spark ignition internal combustion engines having at least two inlet valves per cylinder and, in a preferred aspect of the invention, engines having 2 inlet and 2 exhaust valves per cylinder head, without the use of additional moving parts.
According to one aspect of the present invention there is provided an internal combustion engine comprising a cylinder head having formed therein first and second intake valves for admitting combustion charge into a combustion chamber and one or more exhaust valves for expelling combustion products from the combustion chamber, the intake valves being disposed on one side of the cylinder head and the exhaust valve or valves being disposed on an opposite side of the cylinder head, each of which valves comprises a port and an associated valve head permitting opening and closing of the port, characterized in that the engine is adapted so as to admit a greater flow of combustion charge through the first intake valve as compared to the second intake valve, the differential flow between the two intake valves imparting a swirl to the combustion charge as it enters the combustion chamber, the swirl direction generally corresponding to circumferential charge flow in the combustion chamber from the first intake valve, past the exhaust valve or valves and thence to the second intake valve.
The invention can be applied to any engine which has plural intake valves, i.e. 3 or more valves per cylinder head. This includes three valve per cylinder engines in which there are two intake valves and a single exhaust valve. However the invention is likely to find most common application in 4 valve engines. Hence, in a preferred aspect of the invention there are first and second exhaust valves and the engine is a four valve per cylinder head engine.
Typically an intake conduit carrying mixture to at least the first intake valve is configured to provide an airflow momentum which directs mixture into the cylinder head from the first intake valve across the cylinder to an opposite side of the cylinder head under the first exhaust valve.
One benefit of introducing or enhancing swirl is improved low speed torque, an area in which prior art 4-valve engines are notably deficient.
The engine may be adapted to produce a differential flow by the first intake valve""s port and valve head combination being shaped and configured to admit a greater flow of charge than the second valve""s port and valve head combination, and/or the second intake valve""s port and valve head combination being shaped and configured to admit a relatively reduced flow of charge.
The flow rate through the first intake valve may be increased by the engine being configured so that a curtain area of the first intake valve when open is larger than a corresponding curtain area of the second intake valve when open.
In one embodiment of this aspect of the invention, the shaping and configuring comprises forming the first intake valve with a larger port bore and valve head diameter than those of the second intake valve. A charge admitted via the smaller intake valve will have the direction of its motion influenced by the higher flow from the larger intake valve. In a 4 valve configuration this tends to turn the airflow admitted by the smaller valve away from the distal second exhaust valve. As a consequence, the likelihood of unburnt combustion charge immediately exiting via the exhaust valve during the open valve overlap period is reduced. This improves engine efficiency because less un-burnt fuel is expelled from the combustion chamber during induction of the fuel charge. The differential in valve port bores will depend upon the particular engine characteristics. However, a preferred range is exemplified by the first inlet valve having a diameter up to 70% larger than the second inlet valve.
In another aspect of the invention there is provided a method of modifying a conventional engine by re-sizing the intake valves and/or the exhaust valves, the re-sizing causing one intake valve to be larger than the other and/or one exhaust valve to be larger than the other, thereby to produce the swirl effect in the working engine as hereinbefore described. The re-sizing may comprise insertion of differentially-sized valve seat inserts and replacement of the standard valve heads with valve heads having a diameter corresponding to re-sized valve seats. Enlargement of the valve ports may be achieved by reboring.
In another aspect of the invention, the engine is adapted by one charge conduit to the first intake valve having a shallower angle of approach to the valve than another charge conduit to the second intake valve, the said one charge conduit thereby directing charge flow admitted through the first intake valve laterally across the combustion chamber and the relatively steep another charge conduit thereby directing charge flow admitted through the second intake valve downwards into the combustion chamber.
Shaping of the conduit (or throat) may be used alone, or preferably to enhance the basic action developed by the differential intake port flow rates. For example, if the conduit to the larger intake port has a shallower approach than the conduit feeding the smaller intake valve, fuel/air charge is encouraged to be admitted into the cylinder head laterally across the roof of the cylinder head.
The conduit to the smaller intake port may have a steeper approach. This encourages greater flow downward into the combustion chamber. This more downward and lesser flow volume from the smaller intake port imparts a tendency in the charge flow out of the smaller intake port to turn away from the distal exhaust valve. This further reduces any tendency for cross over loss by spilling of the charge out of the proximal first or distal second exhaust ports during overlap.
In the 4 valve configuration, the first exhaust valve may have a smaller port bore and valve head diameter than the second exhaust valve. Since the smaller first exhaust valve is directly opposite the large intake valve, there is less opportunity for the incoming charge to exit via the exhaust valve than if the first exhaust valve were the same size or larger than the second exhaust valve. As combustion charge spirals into the combustion chamber it will tend to pass over the combustion chamber side of the distal second exhaust port""s valve head, thereby avoiding being drawn out of that exhaust valve when the exhaust valve is open during valve opening overlap.
It has been found that further swirl motion enhancements can be achieved by shaping of the combustion chamber roof. A conventional prior art chamber roof configuration is shown in FIG. 12. Some combustion chambers may have even simpler forms, and the most basic in common usage is generally according to that shown in FIG. 13. None of these chamber forms, or those that are essentially hybrids of those shown in FIGS. 12 and 13, is capable of improving mixture motion beyond natural tumble. It is an object of the present invention to provide combustion chamber shaping which does enhance swirl and reduce cross flow.
Hence, according to another aspect of the invention a cylinder head wall portion which forms a roof of the combustion chamber may accommodate therein the intake and exhaust valves, which roof is provided with shrouding around the first intake valve, the shrouding providing a relatively broad flow path out of the first intake valve at a region of the first intake valve proximal to the exhaust valve or valves and a relatively narrow flow path out of the first intake valve at a region of the first intake valve proximal to the second intake valve, the charge flow thereby being encouraged in the swirl direction and discouraged in an anti-swirl direction.
In yet another aspect of the invention there may be provided an engine as hereinbefore described wherein a cylinder head wall portion which forms a roof of the combustion chamber accommodates therein the intake and exhaust valves, which roof is provided with shrouding around the second intake valve, the shrouding providing a relatively broad flow path out of the second intake valve at a region of the second intake valve proximal to the first intake valve and a relatively narrow flow path out of the second intake valve at a region of the second intake valve proximal to the exhaust valve or valves, the charge flow thereby being encouraged in the swirl direction and discouraged in an anti-swirl direction.
By applying a moderate degree of shrouding the free path of the charge passing through the port can be constricted or opened. This effectively modifies the curtain area to induce a differential flow through an individual valve, which leads to the development or enhancement of swirl. Naturally where the curtain area is relatively large, i.e. where the shrouding clearance is greatest, flow is encouraged. Where the curtain area is reduced, i.e. clearance is low, flow is reduced. Hence overall flow develops in the direction of greater clearance, developing a swirl.
Advantage of the same effect may be taken at the smaller second intake valve. This effect is produced in the same manner as the larger first intake to influence mixture flow in the swirl direction.
Beyond enhancing swirl, the motion imparted to the incoming charge by the valve shrouding tends to reduce cross over loss during the overlap period at the end of the exhaust stroke and the beginning of the intake stroke. Because a smaller exhaust valve faces the predominant flow from the larger first intake valve, so there is less exhaust valve curtain area directly aligned through which incoming charge may exit the chamber. As the incoming charge enters the cylinder it spirals down the bore of the combustion chamber. As such the charge has a tendency to pass over the combustion chamber side of the second, larger exhaust port rather than exit through the curtain area between the valve head and valve seat in the port.
Further reductions in cross flow can be achieved by providing a step in the combustion chamber roof portion that separates the intake and exhaust valves. The step may be formed by recessing of the exhaust valve or valves.
Hence, in another aspect of the invention there may be provided an engine as hereinbefore described wherein respective circumferential surfaces of the intake and exhaust valve ports form valve seats for the associated valve heads, wherein the valve seat of the first exhaust port is recessed into a cylinder head roof relative to the valve seat of the first intake port so that the valve seat of the first exhaust port is axially offset in the cylinder head with respect to the valve seat of the first intake port. The recess forms a step that shields the exhaust valve from the entering fuel charge flow. Hence, for cross over to take place the charge would have to follow a tortuous path, which is not therefore favoured.
In a further aspect of the invention there may be provided an engine as hereinbefore described wherein respective circumferential surfaces of the intake and exhaust valve ports form valve seats for the associated valve heads, wherein the valve seat of the second exhaust port is recessed into a cylinder head roof relative to the valve seat of the second intake port so that the valve seat of the second exhaust port is axially offset in the cylinder head with respect to the valve seat of the second intake port.
In another aspect of the invention there may be provided an engine as hereinbefore described, characterized in that a deflecting feature is formed on a cylinder head wall portion separating one of the intake valves from one of the exhaust valves, the deflecting feature serving to deflect the flow of combustion charge entering the combustion chamber via the intake valve downwards into the combustion chamber and away from the exhaust valve.
As the fuel/air charge enters the chamber it is turned to direct it in a more downward direction than would be the case without the deflector portion of the chamber being present. Additionally, because of the stepped form of the chamber, the exhaust valve is recessed from the level of the intake valve and offset with respect to the path of mixture flow in through the intake, so the charge entering the cylinder is less likely to exit out of a still open exhaust port. Preferably the extent of the exhaust port recessing relative to the intake port would be between approximately 20 and 50% beyond the normal countersink present around a valve seat. In one aspect of the invention the exhaust port is recessed to between 5 and 20% of the exhaust valve port/head diameter.
In yet another aspect of the invention the engine is adapted to provide greater flow through the first intake valve by the provision of differential valve timing. Hence according to yet another aspect of the invention there may be provided an engine as hereinbefore described wherein the engine is adapted to provide greater flow through the first valve by the provision of differential valve timing means which act to open the first intake valve before the second intake valve so that a low pressure region is created adjacent the first intake valve, which low pressure region draws subsequent charge flow from the second intake port toward the first intake port, thereby imparting a swirl to the charge entering the combustion chamber.
In another aspect of the invention a valve opening mechanism may be configured to open the first intake valve head a further distance than the second intake valve head, thereby increasing the first intake valve""s curtain area to induce engine swirl. The configuration may comprise providing the first intake valve with larger actuation cams than actuation cams of the second intake valve.
Differential valve timing may be achieved by simply altering the relative rotational positions of valve actuation cams for the respective intake valves. In a preferred embodiment the differential timing means is arranged to open the first intake valve before the second intake valve, and to close the second intake valve before the first intake valve.
It is within the scope of the invention to provide a method of improving the efficiency of a conventional engine by a method comprising reconfiguring the conventional engine to correspond with any engine according to the invention as hereinbefore described.